The isomath package makes it possible to typeset upright Greek letters via \mathrm and \mathbf. Is it it possible to force all Greek letters to be typeset upright unless specified otherwise, e.g. to make it so that \(\Phi \Psi\) delivers the same output as \(\mathrm{\Phi \Psi}\)? An ideal solution would be to load isomath with some option and if it optionally could make difference of upper and lower case Greek letters.

Just for reference, if XeLaTeX is an option, the package unicode-math provides an easy solution to this problem through the "math-style" option, documented at section 5 of the manual.
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pmav99Oct 15 '11 at 22:00

add \usepackage{upgreek}, then you can write \upPsi for example
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Andy BarbourJun 2 '14 at 5:01

The command \DeclareMathSymbol is used for assigning a meaning to characters or commands in math mode. The second argument contains the "kind" of symbol; \mathalpha means a symbol that obeys to the alphabet selection commands. The third argument is the alphabet used by default; the fourth is the slot in the font. All the standard declarations are in fontmath.ltx loaded at format creation time.

Notice that these declarations won't work if the default font encoding of the document is T1 instead of OT1, as operators refers to the ordinary Roman document font. One should define a new math alphabet, in this case.

How to do this when the document encoding is T1? One has to define a new symbol font:

Of course cmr can be changed into the document's main font family name, provided it contains Greek uppercase letters in its OT1 version; but it's just a matter of giving the correct slot numbers for the chosen font.

Cheers, it works now! Do you mind including an explanation of what the lines of \DeclareMathSymbol does, i.e. their meaning, in your answer? Also, if there is an isomath solution I'd be interested to learn about it.
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N.N.Oct 5 '11 at 13:10

isomath was written to set the math style to an 'ISO math style', as the author of the package puts it. Essentially, this means that it will be possible to typeset Greek letters
(and non-Greek ones, for that matter) as bold italics, to denote vectors and matrices according to the ISO standard.
It is possible to have upright letters, too.
isomath requires OML font encoding. The isomath documentation (2011-01-14) states:

Currently only the mathdesign package provides upright fonts in OML encoding.

So, this package also needs to be loaded in order to obtain the correct fonts. LaTeX may otherwise display entirely different characters.

An example based on code snippets from the package documentation could be:

Thanks for you answer. How does this answer my question for a way to force all Greek letters to be typeset upright unless specified otherwise, e.g. to make it so that \(\Phi \Psi\) delivers the same output as \(\mathrm{\Phi \Psi}\)?
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N.N.Oct 17 '11 at 8:14

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I'm sorry if I could not give a satisfactory answer. Maybe you can define a new command: \newcommand{\upGamma}{\mathrm{\Gamma}}. Otherwise you need to change the fonts, as @egreg did in his answer. I'm afraid I don't have the skills to propose anything more 'general' that could substitute e.g. \Gamma with \mathrm{\Gamma}. (\renewcommand obviously doesn't work here.)
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Count ZeroOct 17 '11 at 11:13